Issue Archive

Alkyl Pyrocarbonate Electrolyte Additives for Li-Ion Cells

Beneficial properties of films that form on carbon anodes are
enhanced.

Alkyl pyrocarbonates have been found
to be useful as electrolyte additives for
improving the low-temperature performances
of rechargeable lithium-ion
electrochemical cells. The beneficial effects
of these and other additives have
been investigated, along with various
electrolyte formulations, in continuing
research directed toward extending the
range of practical operating temperatures
from the present lower limit of –20
°C down to –40 °C, and even lower if
possible. This research at earlier stages
was reported in a number of NASA Tech
Briefs articles; namely, “Update on Electrolytes
for Low-Temperature Lithium
Cells” (NPO-20407), Vol. 24, No. 1, (January
2000), page 56; “Lithium Alkoxide
Electrolyte Additives for Lithium-Ion
Cells” (NPO-20607), Vol. 25, No. 6 (June
2001), page 52; “Aliphatic Ester Electrolyte
Additives for Lithium-Ion Cells”
(NPO-20601), Vol. 25, No. 6 (June
2001), page 53; and “Ethyl Methyl Carbonate
as a Cosolvent for Lithium-Ion
Cells” (NPO-20605), Vol. 25, No. 6 (June
2001), page 53.

To recapitulate from the cited prior
articles: the loss of performance with
decreasing temperature is attributable
largely to a decrease of ionic conductivity
and the increase in viscosity of
the electrolyte. What is needed to extend
the minimum operating temperature
from –20 °C down to –40 °C is a
stable electrolyte solution with relatively
small low-temperature viscosity, a
large electric permittivity, adequate coordination
behavior, and appropriate
ranges of solubilities of liquid and salt
constituents. Whether the anode is
made of graphitic or non-graphitic carbon,
the surface film acts as a
solid/electrolyte interface (SEI), the
nature of which is critical to low-temperature
performance. Desirably, the
surface film should exert a chemically
protective effect on both the anode
and the electrolyte, yet should remain
conductive to lithium ions to facilitate
intercalation and deintercalation of
the ions into and out of the carbon
during discharging and charging, respectively.

One previously reported optimized
electrolyte formulation is a 1.0 M solution
of LiPF6 in a ternary solvent
that consists of equal volume parts of
ethylene carbonate (EC), dimethyl
carbonate (DMC), and diethyl carbonate
(DEC). Also previously reported
is the use of quaternary additives
to this baseline optimized
formulation to enhance low-temperature
performance.

The present alkyl pyrocarbonate additives
(see figure) to the baseline optimized
electrolyte formulation promote
the formation of protective and conductive
SEIs on carbon anodes. The formation
of such SEIs is believed to be facilitated
by products (e.g., CO2) of the
decomposition of these additives.
These decomposition products are believed
to react to form Li2CO3-based
films on the carbon electrodes. The improvement
(relative to the baseline formulation)
in interfacial properties resulting
from the use of these additives is
more evident at low temperature,
where enhanced kinetics of intercalation
and deintercalation of Li, higher
ionic transport across SEIs, and increased
discharge capacities with low
overpotentials are observed. Also, the
SEIs that form in the presence of these
additives are more stable toward any
further reduction of the electrolyte and
thus more stable against growth to
greater thicknesses; hence, they contribute
to the cycle lives of the anodes.

This work was done by Marshall Smart,
Ratnakumar Bugga, and Subbarao Surampudi
of Caltech for NASA’s Jet Propulsion
Laboratory. For further information, access
the Technical Support Package (TSP)
free on-line at www.nasatech.com/tsp
under the Materials category.

In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to
Intellectual Property group
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240
Refer to NPO-20775, volume and number
of this NASA Tech Briefs issue, and the
page number.

This Brief includes a Technical Support Package (TSP).

Alkyl Pyrocarbonate Electrolyte Additives fro Li-Ion Cells (reference NPO-20775) is currently available for download from the TSP library.

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